Typically, superhydrophilic surfaces have a water contact angle of less than 25° and water-capturing surfaces retain water as a uniform film having a thickness of millimeters.
Many techniques have been developed to prepare superhydrophilic surfaces. See I. P. Parkin et. al., J. Mater. Chem. 2005, 15, 1689. For example, superhydrophilic surfaces are obtained by UV-irradiation of oxide semiconductor films such as TiO2 and ZnO. In this method, the superhydrophilicity, induced by photon-generated short-lived charges, gradually disappears without continuous UV illumination. See, e.g., X. M. Li et. al., Chem. Soc. Rev. 2007, 36, 1350, A. Lafuma et. al., Nat. Mater. 2003, 2, 457.
A method has been devised for producing water-capturing surfaces that mimic the water harvesting wing surfaces of the Namib Desert beetle. See L. Zhai, Nano Lett. 2006, 6, 1213. This method is not suitable for large-scale production as it involves layer-by-layer multi-step patterning and deposition of both hydrophilic and hydrophobic components.
Of note, superhydrophilic surfaces do not necessarily possess water-capturing capacity.
Surfaces that are superhydrophilic and/or water-capturing have many industrial applications. Superhydrophilicity prevents fog formation, as condensed water spreads across a superhydrophilic surface. On the other hand, water-capturing surfaces can used to draw water from dew in arid areas. A surface that possesses both superhydrophilicity and water-capturing capacity is ideal for use in a solid-state supercapacitor.
There is a need for cost-efficient methods of preparing enduring superhydrophilic and/or water-capturing surfaces.